The present invention relates to a chair backrest device comprising the flexible back support supporting the back of a sitting person in a backrest frame.
Such a chair backrest device is disclosed, for example, in JP2002-125797A, JP2005-160558A, particularly in FIGS. 10-17, and JP2001-128785A.
In the backrest shell of the backrest device in JP2002-125797A and JP2005-160558A, a number of openings are formed all over the backrest shell, which is flexed rearward when it is pressed by the back of a person. But the thickness of the backrest shell is almost equal, and the whole backrest shell is not effectively flexed rearward along the shape of the back, so that seating comfort is not obtained.
To overcome the disadvantage, the backrest shell can be made thinner for easier flexing, but the backrest shell decreases in strength and rigidity, so that durability is decreased and seating comfort is likely to get worse due to too much flexing.
In the backrest device in JP2001-128785A, in the upper part of the backrest shell, there are two vertically elongate openings which engage with the projections on the upper part of the backrest frame to enable the backrest shell to move up and down. Furthermore, in the lower part of the backrest frame, there are holding portions projecting forward and having grooves. On the backrest shell, there are lobes which engage with the holding portions and which is pressed downward on the grooves, so that the backrest shell is supported by the backrest frame to enable the backrest shell to flex rearward.
However, in the support structure for the backrest shell, the right and left sides of the backrest shell supported by the backrest frame does not turn around a vertical axis, so that the whole backrest shell does not flex rearward effectively along the shape of the back of a person.